The present invention relates to semiconductor devices, and particularly to a semiconductor device using resistor elements such as polysilicon resistors. More particularly, the present invention relates to a configuration which inhibits the substrate voltage dependences of resistor elements such as polysilicon resistors.
In a semiconductor circuit device, resistor elements are used in various portions thereof. To implement a desired operation characteristic of a circuit, a resistor element is required to precisely retain the resistance value thereof. Typically, when a diffused resistor using the resistance of an impurity layer (diffusion layer) is used as a resistor element, the size of a depletion layer differs according to the potential difference between the diffused resistor and a well region where the diffused resistor is formed, and the resistance value of the diffused resistor varies in accordance therewith.
A configuration aiming at inhibiting the substrate voltage dependence of such a diffused resistor is shown in Japanese Unexamined Patent Publication No. 2001-168651 (Patent Document 1). In the configuration shown in Patent Document 1, an insulating film is formed over an epitaxial layer, and resistor elements formed of polysilicon or the like are disposed over the insulating film. In a layer overlying the resistor elements, a shield film for preventing noise from another wiring or the like is disposed. When the resistor elements are coupled in series, i.e., when the resistor elements are used as, e.g., the input resistor and feedback resistor of an operational amplifier (op-amp), a shield layer is coupled to one end of each of the resistor elements such that the voltages between the shield layer and the resistor elements have the same polarity. The epitaxial layer is brought into a floating state.
In Patent Document 1, the same polarity is given to each of the voltages between the shield layer and the resistor elements of the input resistor and feedback resistor of the op-amp to cause equal changes in the respective resistance values of the input resistor and the feedback resistor. By maintaining the resistance ratio therebetween at a given value, it is intended to hold constant the amplification factor of the op-amp. In addition, by providing a configuration in which a substrate bias voltage is not applied to the epitaxial layer, it is intended to eliminate the need for a region and wiring for the application of a substrate bias, and thereby implement simplified wiring and a smaller-size resistor circuit.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2001-168651